The synthesis of triglycerides via the glycerol-3-phosphate pathway is inhibited by either 2-monoglycerides or their 2-monoether analogs in adipose and intestinal tissue. The inhibition is specific for the acylation of glycerol-3-phosphate since other microsomal enzymes are not inhibited. The 2-monooleyl ether was shown to be the most potent inhibitor of triglyceride biosynthesis in the intestinal mucosa. The subcellular distribution of the enzymes monoglyceride hydrolase and acyl-CoA-sn-glycerol-3-phosphate-acyl transferase has been examined in adipose and intestinal tissue employing marker enzymes. The results suggest the hydrolase is present in the lysosomal fraction and contrary to published reports the adipose tissue acyl transferase is a microsomal enzyme. Triglyceride biosynthesis via the monoglyceride pathway in the intestinal mucosa increases as the fetus approaches term and decreases in the adult when triglycerides are removed from the diet. These observations and previous investigations suggest that triglyceride biosynthesis can be modified depending on availability of the various substrates. Phosphatidic acid phosphohydrolase is involved in phosphatidylcholine biosynthesis in the CDP choline pathway. It has a four-fold increase in activity in fetal rabbit lung subcellular fractions. This occurrance on the 27th. day is one day prior to the observed increase in surfactant concentrations and suggests this enzyme plays a role in fetal lung maturation.